402 SCIENTIFIC WRITINGS. 



themselves in volcanoes. English physicists, among 

 them Sir William Thomson, now Lord Kelvin, have 

 opposed this basis of the theory of the earth's formation 

 with weighty arguments. Lord Kelvin has declared 

 that the whole terrestrial body must be more solid than 

 glass -hard steel, as calculation proves that its surface 

 would otherwise participate in the tidal movement 

 produced by the attraction of sun and moon, conse- 

 quently an independent ocean-tide could not then occur. 

 J. Thomson has supported this calculation by a physical 

 consideration, which o*oes to show that the fusing tern- 



' O o 



perature of bodies, which expand on solidification, is 

 lowered by pressure, but of bodies, which contract 

 on solidification, is heightened by pressure. Now since 

 the silicates, as he infers, contract on solidification 

 about 20| , the pressure increasing with the depth 

 would not allow the rock masses to fuse in spite 

 of the heightened temperature, but make them still 

 more solid. 



It is remarkable that these diametrically opposed 

 views on the nature of the earth's crust should have 

 been before the world for years without giving rise 

 to violent controversies, although the question at issue 

 affects the very basis of practical geology. The 

 geologists, as already mentioned, for the most part 

 maintain the theory of a crust floating on a fluid or 

 gaseous nucleus, and the mathematicians cling to Lord 

 Kelvin's theory of a solid nucleus, without troubling 

 themselves much about the difficulties in the way of 

 explaining the actual formation of the surface! 



